Gas (e.g., CO.sub.2) lasers for medical use have been manufactured with a control system which adjusts laser operation parameters during an initial test operation with the output beam blocked. A totally reflective mirror is directed to a thermal average power sensor. The output of the power sensor is digitized and provided to a control processor. Pulse repetition rate is derived by the processor from control panel selections, by a user (e.g., a doctor) of power and energy. Pulse width is determined by the processor to achieve the desired pulse energy. These initially determined laser operation parameters are used during treatment without change. However, gas (and, in particular, CO.sub.2) lasers vary output by approximately plus or minus 10% over time and temperature ranges, for a constant input.
One attempt to solve this problem is described in U.S. Pat. No. 4,950,268, issued Aug. 21, 1990. With reference to FIG. 4 of U.S. Pat. No. 4,950,268, a laser system operating in a pulsed-mode employs measured output beam pulse energy as a feedback signal in a laser power control circuit. When the measured energy of an output beam pulse exceeds a preset value, op amp 42 (of FIG. 4 of U.S. Pat. No. 4,950,268) emits a control signal to reset AND gate 32. In response to the control signal, the laser power supply is "squelched." However, the system of U.S. Pat. No. 4,950,268 is subject to the following serious disadvantage: the laser may (undesirably) emit significant energy after op amp 42 emits a control signal to squelch the power supply, because of stored energy in the lasing medium and inherent time delays occurring during feedback signal generation and processing of the control signal.